Defrost control means for refrigerating systems



J. LJEBERMANN ETAL 3,383,877

5 Sheets-Sheet 1 DEFROST CONTROL MEANS FOR REFRIGERATING SYSTEMS FiledMay 10, 1966 May 21, 1968 H m NZ. 0 N N N T R N M R 0 R T W 1B- L A UE MO HR OE IUJ Y B J. LIEBERMANN ETAL 3,383,877

May 21, 1968 DEFROST CONTROL MEANS FOR REFRIGERATING SYSTEMS 5Sheets-Sheet Filed May 10, 1966 Fig. 6

INVENTORS JOHN LIEBERMANN JEROME L LORENZ W WW4! m y 1968 J. LIEBERMANNETAL 3,383,877

DEFROST CONTROL MEANS FOR REFRIGERATING SYSTEMS 5 Sheets-Sheet 5 FiledMay 10, 1966 W and A77 R/VEYS United States Patent corporation of OhioFiled May 10, 1966, Ser. No. 548,959 4 Claims. (Cl. 62140) The presentinvention relates to control apparatus for automatically defrosting thecooling unit of forced air refrigerating systems to maintain the coolingunit relatively free of frost.

Refrigerating systems for the storage and self-serve display of frozenfoods are commonly used in stores for convenience in the sale of foods,and the foods are maintained well below the freezing temperature bycirculating air thereover which has been chilled by the cooling unit ofthe refrigerating system to about l0 F. During cooling of the air,moisture condenses and freezes on the heat exchange surfaces of thecooling unit, thereby insulating these surfaces from the air andreducing the space through which the air moves across the cooling unit,all of which results in considerable loss in efficiency of the coolingunit. It is desirable that the cooling unit be cleared of frost beforethe frost coats the surfaces of the unit to a depth which preventsadequate chilling of the air. A principal object of the presentinvention, therefore, is the provision of a new and improved controlmeans for initiating and terminating defrosting cycles in the coolingunit of refrigerating systems of the type mentioned.

Another object of the invention is the provision of a new and improvedcontrol means for defrosting the cooling units of refrigeration systemsof the type described which includes relatively simple and reliablecircuitry in which electrical resistance elements are employed assensing means which can be conveniently located in an air duct and atthe air cooling unit to sense frosting conditions and to terminate thedefrost cycles, and which circuitry can be readily adjusted to provideproper defrost cycles according to the operating characteristicsencountered in refrigerating systems of different design details.

Still another object of the invention is the provision of a controlmeans of the character referred to which includes two temperatureresponsive self-heating electrical resistance members, the resistancesof which vary with the temperature thereof and which can be convenientlylocated in the path of the air flowing in a duct of the refrigeratingsystem, one of the resistance members being more or less shielded fromthe air flow so that when the velocity of air in the duct changes due tocollection of frost on the cooling unit the resistance of the unshieldedresistance member changes relative to the other member to effect adefrost cycle, and a third thermally responsive resistance member beingresponsive to the temperature of the cooling unit to sense a frostmelting temperature to effect termination of the defrost cycles, theresistance member being connected in circuit means in such a manner thatthe air velocity resistances are ineffective to initiate a defrost cycleduring the time the temperature of the cooling unit is being reducedfrom a frost melting temperature to the normally maintained sub-freezingtemperatures so that the effect of the velocity sensing resistancemembers in the control circuit is cancelled until the normal temperaturedifferentials of these members has been reestablished.

Other objects and advantages of the present invention will becomeapparent from the following detailed descrip tion thereof and theaccompanying drawings which form a part of the specification and inwhich:

FIG. 1 is a fragmentary perspective view of a frozen food display anddispensing cabinet embodying the present invention;

FIG. 2 is a schematic transverse sectional view of the cabinet shown inFIG. 1;

FIG. 3 is a schematic showing of the refrigerating system employed inthe food cabinet;

FIG. 4 is a schematic fragmentary view of portions of the refrigerationsystem and the defrost control means, applied thereto of the invention;

FIG. 5 is a wiring diagram of control means for the refrigerationapparatus shown in FIG. 3; and

FIG. 6 is a sectional view taken substantially along a plane indicatedby line 66 of FIG. 4.

As representing a prefcrr d embodiment of the present invention, arefrigeration apparatus 10 is shown in FIG. 1 which is particularlydesigned for the display and selfserve dispensing of frozen food stuffs,although the invention could be embodied in many other forms ofrefrigerating apparatus. The apparatus 10 comprises a typical foodstorage and self-serve dispensing type cabinet 11 having an air chillingheat exchange unit 12 and a food storage space 13 which is cooled by airforcefully drawn from the space, directed across the cooling unit andreturned to the space. The food stored in the space 13 is maintained atabout 0 F. or lower, for example, and is accessible through an opening14 across the front of the cabinet.

The cabinet 11 includes a bottom wall 15, vertically extending front andrear walls 16, 17, and end walls 29, 21. The rear wall 17 is integralwith a horizontally extending top wall 22 having a downwardly directedfront portion 23. A partition 24 is located in spaced relation to thebottom, rear and top walls 15, 17, and 22, respectively, and cooperateswith these walls and the end walls 20, 21 to form an air passageway orduct 25 which extends upwardly along the rear of the cabinet andforwardly across the top thereof, terminating in a downwardly facingdischarge opening 26. The width of the duct or passage 25 extendssubstantially the length of the cabinet and the air discharged from theopening 26 across the top of the forward side of the cabinet sweepsdownwardly over the food in the space 13 to maintain the food at about 0F. and is returned to the passage 25 towards an opening 27, across thelower portion of the cabinet. The cooling unit 12 extends transverselyof the duct or passage 25 and across the bottom of the cabinet in thepath of air returned to the passage through opening 27. The unit 12 issuitably supported above the bottom wall 15, and baffles 30, 31 arearranged to direct all of the air moving into the passage through thecooling unit and into the vertical section of the duct 25.

Air is cooled as it is drawn through the cooling unit 12 and is forcedupwardly through the duct 25 and into the space 13 by a plurality ofconstant speed electric motor driven fans 36, only one of which isshown, located in openings formed at intervals along the baflie 3! overthe top of the cooling unit.

Forwardly extending shelves 32, 33 are supported on the partition 24and, with the lower forwardly extending portion 34 of the partition,provide food storage racks in the path of the chilled air descendingfrom the opening 26 to the inlet opening 27, as is indicated by thearrow 35.

The unit 12 comprises the evaporator of a conventionalcompressor-condenser-expander type refrigerating system consisting of anelectric motor driven compressor connected through suitable conduit-swith :a condenser 41 and the evaporator 12, which connections include areversing valve 42 and a refrigerant expansion device 43, all of whichare well-known in the art. The reversing valve 42 is controlled by asolenoid 44 so that when the solenoid is deenergized the valve effects aflow of refrigerant from the compressor to the condenser, thence to theevaporator and return to the compressor to provide normal refrigerationand when the solenoid is energized, the flow of refrigerant is from thecompressor to the evaporator and thence to the condenser, which flow isutilized to provide a defrost cycle which heats the evaporator 12 andmelts ice or frost therefrom, all of which is well-known practice.

The cooling unit 12 is normally maintained well below F. by conventionalthermostatic control means for the compressor which means is not shownas such are well-known.

The cooling unit 12 comprises a serpentine formed pipe as extendingthrough openings in closely spaced vertical fins 46 which form heattransfer surfaces from the pipe for air passing therebetween, whichconstruction is typical.

When the apparatus is in normal operation, air is drawn through thecooling unit 12 by the fans 36, forced upwardly through the passage andis discharged downwardly from the outlet of the duct onto the foodstored on the shelves and returned to the cooling unit. Inasmuch as thechilled air is exposed to atmospheric air at the opening 14, it entrainsmoisture laden air as it passes to the inlet of the air duct, and themoisture condenses and freezes on the fins and coils of the coolingunit, thereby forming an insulation covering thereof and reducing thedimensions of the air passages through the unit. The rate of frostaccumulation varies, depending on the humidity.

To prevent excessive accumulation of frost on the fins of the coolingunit 12, the evaporator is heated in response to an undesirableaccumulation of frost thereon by reversing the how of refrigerantthrough the system by the valve 42 as described hereinbefore.

In accordance with the present invention, the solenoid 44 of thereversing valve 42 is energized and deenergized by control means 50which senses a reduction in the velocity of air in the passage 25,resulting from a build up of frost on pipe 45 and fins 46 of the unit12, and initiates a defrost cycle until the frost is melted, after whichnormal refrigeration is reinstated.

The control means 50 comprises a circuit 53, shown in FIG. 5, whichincludes a solenoid winding 54 of a double-throw relay switch 55 whichcontrols energization of the reversing valve solenoid 44 and the fans36. The fans 36 are energized during the time the valve solenoid 44 isdeenergized and vice versa, so that during defrost cycles no air isforced through the cooling unit and passage 25.

The movable contact 56 of the switch 55 is connected to one line of aconventional 115 v. AC power source L1, L2 and the fixed contact 57 isconnected to one terminal of the fan motors 36, the other terminal ofwhich are connected to L2. The fixed contact 60 of switch 55 isconnected to one terminal of the solenoid 44, the other terminal ofwhich is connected to L2. When relay solenoid 54 is energized, thearmature 61 .of the relay moves con tact 56 to contact 60 to energizethe valve solenoid 44- and open the fan circuit, and when the relaysolenoid is deenergized the armature drops to a position in whichcontact 56 engages contact 57 and breaks contact with the contact 60.

The relay solenoid 54 is energized by a 12 v. unfiltered DC power sourcewhich is provided by a center tap stepdown transformer 62, the primarywinding of which is connected between lines L1, L2 and the terminals ofthe secondary winding of which are connected to a conductor 63 throughdiodes 64, 65 and the center tap of the winding is connected to aconductor 66. The relay solenoid 54 is connected between the conductors63, 66 in series with the emitter and collector of a PNP transistor 67,and a resistor 70, so that the solenoid 54 forms part of an outputcircuit of an amplifier, described fully hereinafter. When thetransistor 67 is turned on and off the solenoid S4 is energized anddeenergized. Preferably, a diode 68 is connected in parallel with thesolenoid coil 54 to permit stable coil operation on the unifiltered DCsupply and to provide a discharge path for the coil energy stored duringthe on cycle.

The input of the amplifier for controlling the solenoid 54 comprises anNPN transistor 71, the collector of which is connected with a junction72 between the base of the transistor 67 and a resistor 73 connected toconductor 63. The emitter of transistor 71 is connected to the slidingcontact of a potentiometer 74, the resistance member 75 of which isconnected in series with a resistance 76 connected to the junction ofthe resistance 7 8 with the emitter of the transistor 67, and a resistor77 connected to conductor 66.

The base of the transistor 71 is biased by alternative voltage dividernetworks, one of which is responsive to a decrease in air velocity inthe passage 25 and includes a junction 80 between two wire woundresistors 81, 82 connected in series between the conductors 63, 66. Thesecond network is responsive to the temperature of the cooling unit 12and includes a junction 83 between two resistors 84, 85 connected inparallel between conductor 63 and junction 83, and a third voltagedivider circuit comprising a junction 86 between resistor 87, rheostat90, and a thermistor 91, which are series connected between conductors63, 66.

The base of transistor 71 is connected with junction 80 through amovable contact 92 of a double throw switch having fixed contacts 93,94. The movable contact 92 is actuated by the armature 61 and when therelay solenoid 54 is deencrgized contact 92 engages contact 93 toconnect the base of the transistor 71 with the air velocity sensingnetwork at junction 80 and removes resistor 84 from circuit. A diode 95is interposed between the contact 93 and junction 86 and preventscurrent flow to the base of transistor '71 and provides voltage andtemperature compensation for the base emitter voltage drop of transistor71.

The base of the transistor 71 is disconnected from junction 80 when thesolenoid 54 is energized by the shifting of contact 92 from contact 93to contact 94, so that the network including thermistor 91 is theneffective to control the transistor 71 rather than the nework comprisedof the resistors 81, 82. A diode 96 is interposed between the junctions83, 86 and prevents flow of current from junction '86 to the base oftransistor 71.

With'the exception of the resistances 81, 82 and the thermistor 91, thecircuit 53 is mounted in a suitable casing 97, which may be convenientlylocated on the apparatus 10.

The resistances 81, 82 are of the wire wound self heating type and arepositioned within cylindrical passages 108, 181 respectively formedthrough a block 102 of insulating material, such as a plastic, which issuitably attached to the upper forwardly extending portion of thepartition 24 of the air duct 25 so that the passages extend in thedirection of air movement through the duct. The passage 101 is blockedby a disc 103 positioned in one end thereof so that no air is circulatedin contact with resistor 82 while an appreciable volume of air flows incontact with the resistor 81 and tends to reduce its temperatureaccording to the rate of air flow. The leads 105 for the resistors 81,82 and for the junction 80 extend from the block 102 to a conventionalplug-in type connector 106 which is plugged into a suitable plugreceptacle in the wall of the casing 97 but not shown in detail.

The electrical resistance of resistors 81, 82 increase with increase intemperature thereof and are adapted to become heated above atmospherictemperatures when the transformer 62 is energized. Both resistors 81, 82are identical and during normal operation of the refrigerating system,the chilled air flowing through the passage of block 102 cools theresistor 81 to a degree well below that of resistor 82 which has no airflowing thereabout and accordingly retains more of the heat generated bythe current flowing therethrough. As frost and ice accumulate on thefins and coils of the cooling unit 12, the volume of air flowingtherethrough is reduced and the corresponding reduction in the velocityof m'r passing through passage 100 of block 102 results in a rise intemperature of the resistor 81 through a greater increment than that ofresistor 82 in the blocked passage 101.

The thermistor 91 is positioned in a bracket 107 suitably attached tothe cooling unit 12 preferably in a location which is last to lose frostduring a defrost cycle and the leads 108 thereof are attached to theplug connector 106. The thermistor 91 is the type which decreases inresistance as the temperature thereof increases.

In the form of the invention disclosed herein the values of the variouscomponents and type designations are as follows:

The operation of the defrost control means is as follows: The compressorcontrolled by the conventional control means mentioned, maintains thetemperature of the cooling unit 12 at about l0 F., and the fans 36operate to circulate air over the unit and into the compartment 13.Switch contact 92 engages contact 93 so that the junction 80 isconnected with the base of transistor 71. When the unit 12 issubstantially free of frost, the air passing about resistor 81 cools theresistor to a temperature considerably below that at which resistor 82is cooled and consequently the resistance of the latter is substantiallygreater than that of resistor 81. Accordingly, the base of transistor 71is negatively biased so that it is held off, as is transistor 6-7, andcurrent flows through resistor 85, switch contacts 92, 93, diode 95,junction 80, resistor 81 to conductor 66. At the normally lowtemperature of the cooling unit 12, the resistance of the thermistor 91is relatively high and in conjunction with the resistances of rheostat90 and resistance 87 biases the cathode of diode 96 more positive thanits anode so that no current flows to the base of transistor 71 frornhevoltage divider network including the thermistor 91.

As frost builds up on the surfaces of cooling unit 12, the rate of airflow through the passage decreases and the air velocity sensing resistor81 is not cooled as eifectively as previously so that its temperatureand resistance increase. This change in resistance is reflected at thejunction 80 which becomes more positively biased and at a pre-set pointin this shift in bias, the base of transistor 71 becomes slightlypositive causing it to start turning on. This results in transistor 67turning on and the increased current in the resistor 70 causes theemitter bias of transistor 71 to become more negative thereby biasingtransistor 71 and subsequently transistor 67 fully on. Current throughthe collector of transistor 67 energizes the relay solenoid 54, causingthe armature to move contact 56 from contact 57 to contact 60, therebyshutting down the fans 36 and energizing the solenoid 44 of thereversing valve 42. Also, contact 92 is moved by the armature fromcontact 93 to contact 94 which disconnects the voltage divider networkcomprising resistances -81, 82 from the circuit and throws resistance 84in parallel with resistance 85, resulting in appreciably increasedforward bias on the base of transistor 71.

Energization of the reversing valve solenoid 44 results in hotrefrigerant being directed into the cooling unit and warming its heatexchange surfaces and melting the frost therefrom. As the temperature ofthe cooling unit increases, the thermistor 91 is warmed, thereby causinga drop in its resistance. At a given temperature at which all frostshould be melted from the cooling unit, determined by adjusting theresistance of the potentiometer 74, the decrease in resistance of thethermistor 91 results in a potential at junction 83 which is negativerelative to the emitter of the transistor 71, which shuts off transistor71, and consequently transistor 67 is turned off, thereby deenergizingthe relay coil 54. Armature 61 drops, closing contact 56 onto contact57, deenergizing valve solenoid 44 to terminate the defrosting cycle ofthe cooling unit and restoring normal refrigeration. At the same time,contact 92 drops onto contact 93, reconnecting the network includingresistors 81, 82 with the base of the transistor 71; however, resistor84 is disconnected from parallel circuit with resistors 85 and 87 sothat the relatively low resistance of the thermistor 91 now drawssufficient current to maintain the base of transistor 71 biased offbecause the resistance of the parallel network comprising the resistors81, 82 is relatively high. When the temperature of the cooling unit andthe thermistor 91 is reduced to about 0 F., the increased resistancethereof with the reduction in resistance of the resistors 81, 82 as theyare cooled by the fiow of air through the passage 100 and about theblock 102 causes the control of the base of transistor 71 to be shiftedto the resistors 81, 82. Thus, on initial operation of the cooling unit12 from room temperature or upon resumption of a refrigeration cyclefollowing a defrost cycle, the unstable thermal condition of theresistors 81, 82 during that time cannot effect a false defrost cycle.

The diode 96 provides voltage and temperature compensation for the baseemitter voltage drop of transistor 71 and effectively removes the airvelocity sensor resistors 81, 82 from the circuit during the time thetemperature of the cooling unit is reduced from above freezing to near 0F.

The rheostat 90 may be used to adjust the network including thermistor91 to set the defrost termination temperature. The resistors 84, 85, and87 are effective to prevent forward bias of the diode 95 and areinstatement of the control of the resistances 81, 82 on the base oftransistor 71 until the temperature of the thermistor 91 at the coolingunit has dropped to about 0 F. By this time the temperatures of thesensing members 81, 82 will be stabilized at their normal differentialby the high velocity air flow so that they are in condition to retaintransistor 71 off until a drop in air velocity is sensed to initiate adefrost cycle as described hereinbefore.

It will be appreciated that the control circuit 53 is relatively simpleand may be formed of durable and relatively inexpensive components. Theair velocity sensing members and the defrost cycle terminating membersare compact and can readily be installed in the refrigerating system.The potentiometer 74 and rheostat 90 are of conventional types which canbe located in the case 95 so as to be accessible for adjustment by atool, such as a screw driver, inserted in openings 109, in the easing 97whereby the sensitivity of the air velocity network and the defrostcycle terminating network can be conveniently and accurately calibrated.

Although but one form of the invention has been shown and described,other forms, modifications, and adaptations thereof may be made, allfalling within the scope of the claims which follow.

We claim:

1. In a refrigeration system for cooling a storage space and comprisingan air cooling heat exchange unit having spaced heat exchange surfaces,blower means operable at a given rate to force air across said surfacesand into the space to be cooled, defrosting means for heating saidsurfaces to remove frost therefrom, control means for said defrostingmeans comprising an electrically actuated control element, circuit meansfor controlling energizetion of said control element and including asource of DC potential, said element being connected across said sourceof DC potential, a pair of self heating resistance members connected inseries across said DC source and comprising a first voltage dividernetwork, means responsive to the temperature of said cooling unit andconnected across said DC source and in parallel with said resistancemembers, means for varying the relative resistance of said resistancemembers in response to a change in the velocity of the air moved throughsaid cooling unit by said blower means, resistance means connected inseries with said temperature responsive means and comprising with saidtemperature responsive means a second voltage divider network,amplifying means for controlling the flow of current through saidcontrol element and including a transistor, means for alternativelyconnecting the base of said transitor to the junction between saidresistance members of said first network and a junction in said secondnetwork, and means responsive to said control element for actuating saidswitch means.

2. In a refrigeration system of the character defined in claim 1 inwhich said temperature responsive means includes a thermistor and saidresistance means connected in series with said thermistor includes arheostat.

3. In a refrigeration system of the character defined in claim 1 inwhich said means for varying the relative resistance of said resistancemembers comprises a body having two air passageways therethrough, a selfheating resistor in each of mid-air passageways, and means restrictingthe passage of air through one of said passageways.

4. In a refrigeration system of the character defined in claim 3 inwhich the last named means comprises a disc-like member having anopening therethrough.

References Cited UNITED STATES PATENTS 3,220,208 11/1965 Oram 621403,222,882 12/1965 Sutton 62-140 XR 3,248,892 5/1966 Sutton 62l563,335,576 8/1967 Phillips 62-156 MEYER PERLIN, Primary Examiner.

1. IN A REFRIGERATION SYSTEM FOR COOLING A STORAGE SPACE AND COMPRISINGAN AIR COOLING HEAT EXCHANGE UNIT HAVING SPACED HEAT EXCHANGE SURFACES,BLOWER MEANS OPERABLE AT A GIVEN RATE TO FORCE AIR ACROSS SAID SURFACESAND INTO THE SPACE TO BE COOLED, DEFROSTING MEANS FOR HEATING SAIDSURFACES TO REMOVE FROST THEREFROM, CONTROL MEANS FOR SAID DEFROSTINGMEANS COMPRISING AN ELECTRICALLY ACTUATED CONTROL ELEMENT, CIRCUIT MEANSFOR CONTROLLING ENERGIZATION OF SAID CONTROL ELEMENT AND INCLUDING ASOURCE OF DC POTENTIAL, SAID ELEMENT BEING CONNECTED ACROSS SAID SOURCEOF DC POTENTIAL, A PAIR OF SELF HEATING RESISTANCE MEMBERS CONNECTED INSERIES ACROSS SAID DC SOURCE AND COMPRISING A FIRST VOLTAGE DIVIDERNETWORK, MEANS REPSONSIVE TO THE TEMPERATURE OF SAID COOLING UNIT ANDCONNECTED ACROSS SAID DC SOURCE AND IN PARALLEL WITH